Mills, who had announced in February that she planned to resign, told staffers of her plans on Wednesday, said Emily Cain, a spokeswoman for the SBA.

Mills had pledged to stay in her position until her successor was ready to be sworn in, but President Barack Obama has not nominated someone to head the SBA. The White House did not immediately respond to a reporter’s request for comment about Mills’ departure and when a successor might be named.

“The agency is working closely with the White House on transition plans and on the appointment of a successor,” Cain said.

It’s not known who will be in charge of the SBA when Mills leaves. The SBA’s second highest-ranking official, Deputy Administrator Marie Johns, resigned from the agency earlier this year. No replacement has been named for her.

Mills has been SBA administrator since April 2009, and joined Obama’s cabinet in January 2012. She has had support from both political parties and from small business advocacy groups for her holistic approach toward helping small business. She has said the government needed to do more than guarantee loans for companies; it also needed to help struggling companies get the counseling they needed in order to qualify for loans.

Under her direction, the agency streamlined its loan process and shortened the amount of time it takes for companies to be granted loans.